Polyester coating compositions in solvent form are well known in the art. These types of resin solutions pose fire and health hazards and are expensive. Curtice, in U.S. Pat. No. 3,310,512, solved the fire and health problems, by teaching an aqueous coating system containing a water dispersible polyester resin having a low acid value.
Curtice reacted an aromatic dicarboxylic acid, such as isophthalic acid with an aliphatic dicarboxylic acid, such as adipic acid, and at least two polyols, one of which must be hydrophilic, to impart water dispersibility to the final resin. The hydrophilic polyol is a polyalkylene glycol, such as polyethylene glycol and polypropylene glycol. The second polyol is selected from cyclic and aliphatic polyhydric alcohols such as neopentyl glycol, trimethylene glycol, glycerine, trimethylol ethane and trimethylol propane. The ratio of dicarboxylic acid:total polyol is 1:0.95 to 1.8, i.e., OH/COOH = 0.95 to 1.8, providing a neutral or basic solution.
In Curtice, all of the reactants are added together and heated in an esterification process, until the acid value drops to below 30. This water soluble polyester is then added to a 98% aqueous amine neutralizing solution, to form a polyester dispersion, having a pH of about 7. This dispersion can be cast as a wet film, dried, and baked at about 120.degree. C., to provide hard, flexible films having good adhesion and solvent resistance properties. These polyesters, while providing a water dispersible system, could however, be improved in terms of increased water solubility, and improved thermal properties upon cure.